Add ViewObserver to View for view updates

ui/views/view.h

Index: ui/views/view.h
diff --git a/ui/views/view.h b/ui/views/view.h
index bb5ebdf78cb1213e16c4dd0cd41c4ad73e510466..5eaa6b61b8b43eae1c4a3339436ab30e7488465b 100644
--- a/ui/views/view.h
+++ b/ui/views/view.h
@@ -74,6 +74,7 @@ class FocusTraversable;
 class LayoutManager;
 class NativeViewAccessibility;
 class ScrollView;
+class ViewObserver;
 class Widget;
 class WordLookupClient;
 
@@ -1000,6 +1001,13 @@ class VIEWS_EXPORT View : public ui::LayerDelegate,
   virtual int GetLineScrollIncrement(ScrollView* scroll_view,
                                      bool is_horizontal, bool is_positive);
 
+  // Observers -----------------------------------------------------------------

[sky, 2016/11/21 16:33:37]

No need for this comment, please remove.

[Sarmad Hashmi, 2016/11/21 17:09:53]

Done.

+
+  void AddObserver(ViewObserver* observer);
+  void RemoveObserver(ViewObserver* observer);
+  bool HasObserver(const ViewObserver* observer) const;
+  void NotifyViewParentChanged(View* view, View* old_parent, View* new_parent);

[sky, 2016/11/21 16:33:37]

Why is this function public? Also, why does it take a view? Can't the view be
|this|?

[Sarmad Hashmi, 2016/11/21 17:09:53]

Made it private. Updated NotifyViewParentChanged to only be called on the |view|
itself (was called on |parent| before). NotifyViewParentChanged and
OnViewParentChanged then only need |old_parent| because |new_parent| ==
View::parent().

+
  protected:
   // Used to track a drag. RootView passes this into
   // ProcessMousePressed/Dragged.
@@ -1575,6 +1583,10 @@ class VIEWS_EXPORT View : public ui::LayerDelegate,
   // so we can't use a scoped_ptr. It's dereferenced in the destructor.
   NativeViewAccessibility* native_view_accessibility_;
 
+  // Observers -------------------------------------------------------------
+
+  base::ObserverList<ViewObserver> observers_;
+
   DISALLOW_COPY_AND_ASSIGN(View);
 };